For example, in electric vehicles, hybrid electric vehicles and the likes, a rechargeable battery (a second battery) is used for supplying electric power to an electric motor that drives the vehicles and accumulating electric energy charged from the electric motor functioning as a generator to recover braking energy or from power sources provided on the ground.
In this case, it is necessary to carry out battery management by always monitoring the state of battery, especially the State of Charge (SoC) to maintain the battery in a good state over long periods.
A current integration method (a Coulomb counting method or a sequential state recording method) and an open-circuit voltage method are known as conventional methods for detecting the SoC. Specifically, in the current integration method, input and output of voltage and current of the battery are recorded as time series data, which is used for calculating the present amount of electric charge by time-integrating the current, and then the Soc is obtained based on the initial amount of electric charge charged in the battery and the full charge capacity. In the open circuit voltage method, the inputted current values and terminal voltage values of the battery are inputted, and using a battery equivalent model, the open circuit voltage values are sequentially estimated as the state quantity of the model. Then the SoC is estimated based on the open circuit voltage.
The methods described above have both merits and demerits: the estimate accuracy for a short time is higher in the former current integration method than that in the latter open circuit voltage method where the SoC is estimated, using the open circuit voltage, but the former needs to always observe, and the error becomes to be accumulated, which deteriorates its accuracy. On the other hand, the latter open circuit voltage method does not need to always observe, but the accuracy is inferior to that of the former current integration method when the change amount of charge in a short time is estimated, because the variation of the open circuit voltage is small relative to the change of the SoC.
One of the conventional battery SoC estimation device is known (for example, see the Patent Document 1), which includes a first electric quantity calculation means, a second electric quantity calculation means, and an offset amount estimation means. The first electric quantity calculation means calculates the SoC of a battery based on an open circuit voltage estimate value estimated based on the discharge-ad-charge current and the terminal voltage of the battery by the open circuit voltage method using an adaptive filter, then calculating the change amount of electric quantity charged in the battery based on the SoC.
The second electric quantity calculation means uses the current integration method to time-integrate the discharge-and-charge current of the battery and calculate the change amount of electric quantity charged in the battery based on the value of integral. The offset amount estimation means estimates the offset amount as the observed-value error of the discharge current measuring equipment based on the difference between the change amounts of the electric quantities respectively obtained by the calculation means. The device improves the estimate accuracy of the inner state of the battery such as the SoC, by suppressing the observed-value error of the discharge current measuring equipment to a small extent by compensating the offset amount to improve the accuracy of the detected current value.
In addition, the other of the conventional SoC estimation methods is known (for example, see the Patent Document 2), which includes a step of obtaining the information on a battery, a step of calculating integrated capacity, a step of judging compensation right-or-wrong, and a step of compensating the integrated value. In the step of obtaining the information on a battery, the information on the battery (current value, voltage value, temperature value) is observed. In the step of calculating integrated capacity, the current integration value is calculated by integrating the compensated current value compensated by using the voltage value. In the step of judging compensation right-or-wrong, the compensation of the current integration capacity is judged right or wrong based on the forward voltage capacity of the battery calculated based on the battery information. In the step of compensating the integrated value, the remaining capacity of the battery by compensating or not compensating the current integration capacity according to the judgment result. The device improves the calculation accuracy of the SoC by compensating the observed error of the battery information.